The invention relates generally to a power generator, and in particular to a reduction of heat dissipation and undesirable voltage differentials in a power generator.
Thermal issues are critical to the design of a high power electrical generator and can serve as limiting factors in generator operation. A typical design of a high power electric generator includes a rotor having rotor windings rotatably disposed inside of a stator having stator windings. The rotation of the rotor induces an electromagnetic field in the stator, which electromagnetic field in turn induces a current in, and voltage drop across, the stator windings. However, the electromagnetic field also induces eddy currents in the stator, which is magnetically and electrically resistive. The eddy currents cause the dissipation of energy in the stator in the form of heat and impose a thermal constraint on the operation of the generator.
In order to improve generator efficiency and reduce generator size, generator manufacturers are constantly endeavoring to improve the thermal performance of the generator. For example, a prior art design of a high power electrical generator 100 is illustrated in FIGS. 1, 2, and 3. FIG. 1 is a cross-sectional view of generator 100 from an isometric perspective. FIG. 2 is a cut-away view of generator 100 along axis 2xe2x80x942. As shown in FIGS. 1 and 2, electrical generator 100 includes a substantially cylindrical stator 102 having a stator core 104 and housing a substantially cylindrical rotor 110. Multiple circumferentially distributed and axially oriented keybars 118 are coupled together at each of a proximal end and a distal end by one of multiple flanges 204 (not shown in FIG. 1). Each keybar 118 is coupled to an outer surface of stator 102. The multiple keybars 118, together with the multiple flanges 204, form a keybar cage around the stator 102.
An inner surface of stator 102 includes multiple stator slots 106 that are circumferentially distributed around an inner surface of stator 102. Each stator slot 106 is radially oriented and longitudinally extends approximately a full length of stator 102. Each stator slot 106 receives an electrically conductive stator winding (not shown).
Rotor 110 is rotatably disposed inside of stator 102. An outer surface of rotor 110 includes multiple rotor slots 114 that are circumferentially distributed around the outer surface of rotor 110. Each rotor slot 114 is radially oriented and longitudinally extends approximately a full length of rotor 110. An air gap exists between stator 102 and rotor 110 and allows for a peripheral rotation of rotor 110 about axis 130.
Each rotor slot 114 receives an electrically conductive rotor winding (not shown). Each rotor winding typically extends from a proximal end of rotor 110 to a distal end of the rotor in a first rotor slot 114, and then returns from the distal end to the proximal end in a second rotor slot 114, thereby forming a loop around a portion of the rotor. When a direct current (DC) voltage differential is applied across a rotor winding at the proximal end of rotor 110, an electrical DC current is induced in the winding.
Similar to the rotor windings, each stator winding typically extends from a proximal end of stator 102 to a distal end of the stator in a first stator slot 106, and then returns from the distal end of the stator to the proximal end of the stator in a second stator slot 106, thereby forming a stator winding loop. A rotation of rotor 110 inside of stator 102 when a DC current is flowing in the multiple windings of rotor 110 induces electromagnetic fields in, and a passage of magnetic flux through, stator 102 and the loops of stator windings. The passage of magnetic flux in turn induces an alternating current in each stator winding and eddy currents arid magnetic and resistive losses in stator 102.
FIG. 3 is a side view of a cross-section of generator 100 and illustrates a coupling of magnetic flux 302 from rotor 110 to stator 102 as the rotor rotates inside of the stator. Magnetic flux 302 generated by a rotation of rotor 110 couples to and passes through the surrounding stator 102. Magnetic flux 302 induces a flow of multiple eddy currents in the magnetically and electrically resistive stator 102, which currents cause energy dissipation and heat generation in the stator that poses a thermal constraint on the operation and capacity of generator 100. As a result, generator designers are always seeking improved methods of thermal management for power generator stators.
One known thermal management technique is the construction of stator core 104 from multiple ring-shaped laminations 402. FIG. 4 is a partial perspective of generator of 100 and illustrates a typical technique of constructing stator core 104. As shown in FIG. 4, the multiple ring-shaped laminations 402 are stacked one on top of another in order to build up stator core 104. Each lamination 402 is divided into multiple lamination segments 404. Each lamination segment 404 includes multiple slots 120 (not shown in FIG. 4), wherein at least one slot 120 of each segment 404 aligns with one of the multiple keybars 118. Each keybar in turn includes an outer side 124 and an inner, or locking, side 122 that mechanically mates with one of the multiple slots 120. Stator core 104 is then constructed by sliding each lamination segment 404, via one of the multiple slots 120, into the keybar cage formed by the multiple keybars 118. The coupling of one of the multiple slots 120 of a lamination segment 404 with a locking side 122 of a keybar 118 affixes each lamination segment 404, and thereby each lamination 402, in position in stator 102. By building stator core 104 from stacked laminations, as opposed to constructing a solid core, circulation of a current induced in stator 102 is limited to a lamination, thereby restricting current circulation and size and concomitantly reducing stator heating.
The above thermal management technique does not fully address thermal problems caused by a xe2x80x9cfringingxe2x80x9d of magnetic flux at each end of stator 102. As illustrated in FIG. 3, the xe2x80x9cfringingxe2x80x9d 304 of magnetic flux at each end of stator 102 results in a number of flux lines 302 axially, or normally, impinging upon each end of stator core 104 and upon the multiple flanges 204. A result of the fringing magnetic flux 304 is a greater flux density at each end of stator core 104 as compared to more centrally located portions of the stator core. The greater flux density at each end of stator core 104 results in increased eddy currents and greater heat dissipation in the laminations of stator core 104 near the ends of the stator, as opposed to more centrally located laminations. The fringing effect also results in increased eddy currents and greater heat dissipation in each flange 204.
In order to combat a buildup of heat at each end of stator 102 due to fringing magnetic flux 304, an inner surface of stator core 104, at each end of the stator core, is radially stepped away 202 from rotor 110, as shown in FIGS. 2 and 3. By increasing the distance between rotor 110 and stator core 104 at each end of the stator core, an amount of flux axially impinging upon each end of the stator core is reduced. However, the stepping of the ends of stator core 104 away from rotor 110 is only a partial solution to the stator core heat dissipation problem presented by xe2x80x9cfringingxe2x80x9d and does not address the problem of heat dissipation in the multiple flanges 204.
A portion of the fringing magnetic flux 304 also impinges upon the ends of each of the multiple keybars 118. The impinging of fringing magnetic flux upon an end of a keybar 118 can produce an uneven coupling of flux into each keybar, with a greater flux density at a keybar end than in more centrally located portions of the keybar. The uneven coupling of flux can produce keybar voltages and keybar currents in each keybar 118. In xe2x80x9cturn, the existence of keybar voltages in each keybar 118 can produce keybar voltage differentials between keybars, which voltage differentials can be transmitted to the lamination segments 404 coupled to the keybars. When a voltage differential is transmitted to adjacent lamination segments 404, the voltage differential can cause arcing between the adjacent segments, overheating in stator core 104, and reduced generator 100 performance. The arcing can also create localized heating in stator core 104, causing lamination segments 404 and lamination rings 402 to fuse together. Such fusing can spread quickly in generator 100 as the lamination segments 404 and lamination rings 402 short circuit to each other, resulting in damage to the generator.
Therefore, a need exists for a method and apparatus for further reducing the heat dissipated in the ends of a stator core and in a flange and for providing for a more uniform coupling of flux into a keybar.
Thus there is a particular need for a method and apparatus that reduces the heat dissipated in the ends of a stator core and in a flange and that provides for a more uniform coupling of flux into a keybar. Briefly, in accordance with an embodiment of the present invention, a flux shunt is provided for insertion adjacent to an inner surface of the stator and approximately at an end of the stator and wherein a permeability of the flux shunt is greater than a permeability of the stator core. The flux shunt reduces the amount of magnetic flux impinging in an axial direction upon the flanges and upon ends of the keybars and the stator core. By reducing the impinging flux, the flux shunt reduces the heat dissipated in the ends of stator and further provides for a more even coupling of flux into a keybar.